Young love: Arin Andrews, 16, pictured left, with girlfriend Katie Hill, 18, were both born the opposite sex

To the casual observer, this young couple look just like any other teenagers in love.

But pretty Katie Hill and her boyfriend Arin Andrews share a unique bond - they were both born as the opposite sex.

Katie, 18, spent the first 15 years of her life as Luke, son of a Marine colonel, while Arin, 16, was born a girl called Emerald who excelled at ballet dancing and won beauty contests.

Both struggled with their sexuality all through their childhoods and were teased and bullied but their lives were changed when they both began hormone therapy and later met at a trans support group in Tulsa, Oklahoma, and instantly fell in love.

Katie said: 'All I saw was a handsome guy. We're perfect for each other because we both had the same troubles growing up.

'We're both size five, so we even swap our old clothes our mum's bought us but we hated.

'We look so convincing as a boy and a girl, nobody even notices now. We secretly feel so good about it because it's the way we've always wanted to be seen.'

High-school student Arin undergoes testosterone shots to give him the more masculine shape he's craved his whole life.

Katie, who has legally been a female since she was 15, believes she was born with naturally high levels of oestrogen and was teased as a boy throughout school for having small AA-cup breasts.

An anonymous donor was so touched by her story they paid for her £25,000 ($40,000) gender reassignment surgery just four days after her 18th birthday on May 16, 2012.

Katie added: 'Even from age three, I knew deep down I wanted to be a girl. All I wanted was to play with dolls. I hated my boy body and never felt right in it.

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Boy meets girl, girl meets boy: Arin Andrews, 16, left, was once a girl called Emerald while his girlfriend Katie Hill, 18, right, was born a boy called Luke

Before the transgender treatment: Emerald Andrews, pictured left at the age of four at dance recital in Tulsa, Oklahoma, before she became Arin, and Luke Hill pictured right at the age of five before he became Katie

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Arin recalls a similar experience and says he knew he was a boy inside from his first day at school, aged just five.

'The teachers separated the girls and boys into separate lines for a game,' he said. 'I didn't understand why they asked me to stand with the girls.

Lucky in love: The sweethearts met through a support group for young transsexuals in their hometown in Oklahoma, America

'Girly things didn't interest me, but I was worried what people would think if I said I wanted to be a boy, so I kept it secret.'

Arin's mother Denise, 41, encouraged him to compete in local pageant contests and he became an accomplished female ballet and clogging dancer.

But Arin's secret love was riding motocross bikes with his father Mitch, 42, and doing triathlons and rock-climbing.

'Mum and dad argued when my motocross clashed with my dance schedule,' he said. 'But at age 11, mum gave up. She couldn't win. But I'll never hold it against her. She was just proud of her only daughter.'

Mrs Andrews now supports Arin by helping with his testosterone shots and has vowed to help him pay for breast removal surgery if he can raise half of the £3770 ($6,000) costs himself.

For the time being, Arin hides his chest by 'binding' - tight strapping around the torso use by transgender females-to-males.

Even Arin's little brother Wesley, 11, has even started calling Arin his 'big brother'. 'It makes me so proud,' said Arin.

Arin's determination to live his life as a male comes after years of 'humiliating' bullying throughout school.

'It was horrible,' said Arin. 'I looked like a pretty girl but acted and walked like a boy.

'Everyone started calling me a lesbian. It felt so humiliating. I didn't feel gay at all.

'I started having suicidal thoughts and told my parents I felt confused, but I didn't even know transgender people existed, so it didn't come up.

'They said it was OK to be gay, but put me into therapy for my depression and to help with my confusion. My feelings of being a boy sounded so stupid I didn't even say it in therapy.

'Then a student at my school, a Christian school, told the teacher's office about a girl I was dating.

'They kicked me out because it was a religious school, even though I was a top pupil.

'I started another school and things have been better there. But everything changed when I met Katie.'

'Keeping his true feelings a secret': Emerald Andrews, before she became Arin, at the age of five in Tulsa, Oklahoma

Support: Both Arin and Katie were bullied growing up in Tulsa, Oklahoma, and they want to help prevent that from happening to others

Encouraging education': Both Arin and Katie spent much of their childhood confused about their identity before they discovered the term 'transgender'

The past: Arin poses next to a portrait of himself when he was still known as Emerald

Arin realised he wasn't alone after reading about Katie's transition from male to female in their local newspaper.

A girl at heart: Luke Hill, before he became Katie, aged seven

'All her loneliness and confusion over feeling she was the opposite gender all her life sounded exactly like me,' added Arin.

'It was the moment I first had an explanation for my feelings. I realised I was 'trans' too. I finally had an answer after years of confusion.'

The pair eventually met in a local trans support group and hit it off.

'She was beautiful and looked just like Megan Fox,' said Arin.

'I longed to know who she was and eventually plucked up the courage to do the guy thing and ask for her number.

'She said yes and we started seeing each other.'

Katie had only learned what 'transgender' meant by browsing the internet when she was 15.

'I'd been so unhappy growing up,' she
said.

'As I got older it got worse and I couldn't help being really
girly with a high-pitched voice and high cheekbones.'

Katie's
mother Jazzlyn, 44, and father Randy, 58, a Lieutenant Colonel in the
United States Marine Corps, struggled to deal with their son's confused
feelings at first.

Graduate: Katie Hill, 18, smiles at her family as she waits to get her diploma during Bixby High School's graduation ceremony

Graduating grin: Katie, pictured with her mother Jazzlyn, is the first openly transgender teen to graduate from a high school in Oklahoma

Inspiring: Katie said she wants to share her story and help other transgender children

'My parents were scared stiff and put me through 15 therapist to try and cure my depression,' said Katie. 'None helped because nobody suggested I was trans and I didn't know what it was.

'When I was 15 I found the word 'transgender' while desperately searching the internet and instantly knew it was me.

'Mum was so terrified of losing me.

'I'd once tried to end my life, so she agreed to help me. I owe her everything.

'We made a list of all the things I wanted to do, like wearing bras and panties, getting hormone therapy and maybe one day having gender realignment surgery.

'My brothers and even my grandma Judy were so accepting I was shocked.

'Dad was upset at first because he felt like he was losing his first son.

'Over time he's learned to accept me and now calls me his "daughter", which makes me smile.'

The couple are speaking out about their experience to help raise awareness of trans issues.

'More needs to be done to let people know about transgender issues,' said Katie.

'We both spent years in the wilderness and felt so alone.

'Our parents didn't know how to help because none of us knew being trans was possible.